Single crystals are used in a variety of applications, in particular as scintillation materials in nuclear imaging applications such as positron emission tomography (PET) and Single Photon Emission Computed Tomography (SPECT). Materials used to form single crystals for such applications include sodium chloride, potassium chloride, potassium bromide, lithium fluoride, sodium iodide, cesium iodide and others. Single crystals are also used in semiconductor applications. In such applications, the single crystals are produced from materials such as germanium, silicon, solid solutions of tin, lead tellurides and others.
In order to form a single crystal, a starting material such as sodium iodide, sodium chloride or other suitable material is heated in a crucible to produce a melt. The crucible is positioned in a vacuum chamber having water cooled walls. A rotating vertical rod having a crystal seed on a lower end is then lowered into the chamber until the crystal seed comes into contact with the melt material. The crystal seed is then slowly pulled out of the melt as the rod rotates to thus gradually extract a single crystal from the melt to ultimately form a cylindrical ingot.
The crystallization rate of the melt is dependent on several factors including environmental factors such as a temperature gradient and vacuum level within the chamber. It is desirable that the crystallization rate of the melt be substantially constant in order to avoid the formation of impurities in the single crystal. As the crystal seed is pulled, a partial ingot is formed which is raised above the crucible and extends out of the hot melt. Heat is then radiated from the partial ingot under vacuum conditions toward the cooling walls. This causes rapid cooling of areas of the partial ingot resulting in an undesirable sudden change in the crystallization rate, thus creating conditions which promote the generation of impurities in the single crystal.